In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, the ink receptive regions accept the ink and repel the water. The ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the materials upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the element is considered as positive-working. Conversely, if the non-exposed regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel ink.
Similarly, positive-working compositions can be used to form resist patterns in printed circuit board (PCB) production, thick-and-thin film circuits, resistors, capacitors, and inductors, multichip devices, integrated circuits, and active semiconductive devices.
“Laser direct imaging” methods (LDI) have been known that directly form an offset printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There has been considerable development in this field from more efficient lasers, improved imageable compositions and components thereof.
Thermally sensitive imageable elements can be classified as those that undergo chemical transformation(s) in response to, exposure to, or adsorption of, suitable amounts of thermal energy. The nature of thermally induced chemical transformation may be to ablate the imageable composition in the element, or to change its solubility in a particular developer, or to change the tackiness or hydrophilicity or hydrophobicity of the surface layer of the thermally sensitive layer. As such, thermal imaging can be used to expose predetermined regions of an imageable layer that can serve as a lithographic printing surface or resist pattern in PCB production.
Positive-working imageable compositions containing novolak or other phenolic polymeric binders and diazoquinone imaging components have been prevalent in the lithographic printing plate and photoresist industries for many years. Imageable compositions based on various phenolic resins and infrared radiation absorbing compounds are also well known.
A wide range of thermally-imageable compositions useful as thermographic recording materials are described in GB Patent Publication 1,245,924 (Brinckman). This publication describes increasing the solubility of any given area of the imageable layer in a given solvent by heating the imageable layer by indirect exposure to a short-duration, high intensity visible light or infrared radiation. This radiation can be transmitted or reflected from the background areas of a graphic original located in contact with the recording material. The publication describes various mechanisms and developing materials and novolak resins are included among the aqueous developable compositions that can also include radiation absorbing compounds such as carbon black or C.I. Pigment Blue 27.
Other thermally imageable, single- or multi-layer elements are described for example, in WO 97/039894 (Hoare et al.), WO 98/042507 (West et al.), WO 99/011458 (Ngueng et al.), U.S. Pat. No. 5,840,467 (Katatani), U.S. Pat. No. 6,060,217 (Ngueng et al.), U.S. Pat. No. 6,060,218 (Van Damme et al.), U.S. Pat. No. 6,110,646 (Urano et al.), U.S. Pat. No. 6,117,623 (Kawauchi), U.S. Pat. No. 6,143,464 (Kawauchi), U.S. Pat. No. 6,294,311 (Shimazu et al.), U.S. Pat. No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al.), and U.S. Pat. No. 6,528,228 (Savariar-Hauck et al.), U.S. Patent Application Publications 2002/0081522 (Miyake et al.) and 2004/0067432 A1 (Kitson et al.).
Problem to be Solved
The industry has focused on the need to diminish the solubility of the exposed regions of phenolic binders (dissolution inhibitors) in the imageable layers before exposure and to enhance their solubility after exposure to suitable thermal energy (dissolution enhancers). Several materials capable of increasing the sensitivity of positive-working compositions have been described. All of the described previous dissolution enhancers are of an acidic nature, and include sulfonic acids, sulfinic acids, alkylsulfuric acids, phosphonic acids, phosphinic acids, phosphoric acid esters, carboxylic acids, phenols, sulfonamides and sulfonimides.
These organic acids and cyclic acid anhydrides may be used alone, but they are preferably used in any combination of at least two of them. It is also preferable to use at least one cyclic acid anhydride in addition to at least two organic acids since such a combination would permit the achievement or further improved developing latitude and printing durability.
WO 2004/081662 (Memetea et al.) describes the use of various developability-enhancing compounds of acidic nature with phenolic polymers or poly(vinyl acetals) to enhance the sensitivity of positive-working compositions and elements so that required imaging energy is reduced. Some of the particularly useful poly(vinyl acetals) for such compositions and elements are described in U.S. Pat. Nos. 6,255,033 (Levanon et al.) and 6,541,181 (Levanon et al.).
While the compositions described in the noted Memetea and Levanon et al. publications have provided important advances in the art, there is a continuing need to improve the sensitivity of positive-working compositions and elements even more, and particular in response to infrared radiation.